The objective of this project is to characterize and study the pathogenic potential of an acid fast (AF) bacterium repeatedly isolated from an ultrapure water source furnishing drinking water to some of the Institute's animals. The organism has been identified as Mycobacterium chelonae. Inoculation of euthymic and athymic mice by IV, and euthymic mice by IP, IN, and gavage routes and in the drinking water produced lesions in the liver and, by the IN route, lesions in the lungs. No deaths were observed, even when mice were given cortisone acetate. Mycobacteria were recovered from livers of euthymic mice inoculated 7 days earlier but not thereafter. Granulomas were observed in the liver from post inoculation day (PID) 2 until day 35 when the experiment was terminated. The lesion was greatest in size and reaction in mice killed on PID 10 and 14. By day 35 lesions were well defined resolving granulomas; but, AF bacteria were rare. Acid fast bacteria were observed in livers of athymic mice on PID 50; however, no bacteria were recovered by culture and lesions were minimal or absent. The number of bacteria required to produce lesions will be examined as will ways to increase the likelihood of recovering the organism from the liver. Spontaneous liver lesions will be searched for AF organisms. Isolator maintained mice and rats will be compared to room maintained animals. Other water borne organisms will be examined for their ability to induce comprable liver lesions. This work is significant because of the many similar focal lesions of undetermined origin observed in livers of animals on chronic bioassays conducted under the NIP. Identifying waterborne bacteria and a Mycobacterium sp. In particular as the cause would allow for corrections in management practices.